Electroheating is a method of rapidly heating substances, such as solid or liquid foodstuffs, by passing a current through the material, wherein the material acts as a resistive heater. Such rapid heating methods are disclosed in applicant/assignee's U.S. Pat. Nos. 4,739,140; 5,583,960; 5,636,317 and 5,863,580, the disclosures of which are incorporated herein by reference.
The fluid to be electroheated must be in contact with a large area of the electrode in order to prevent a high current density on the electrode that might lead to arcing. U.S. Pat. Nos. 5,583,960; 5,636,317 and 5,863,580 describe apparatus for increasing the electrode contacting area and thereby reducing the current density. The apparatus includes a narrow tube which terminates at both ends thereof in funnel-like cones. The electrode is the size of the large base of each cone.
A problem exists when attempting to electroheat semi-solid materials, such as coagulated proteins or dough. It is difficult to form good electrical contact between a flat electrode and the semi-solid material. The narrow tube apparatus of the abovementioned patents solves this problem by providing good contact area and low current density at the cone ends. However, although this arrangement provides low current density, it increases the dwell time in the electroheater, since the volume of the cones is much larger than that of the narrow tube. The increased dwell time presents another problem by making it difficult if not impossible to pass the semi-solid material through the electrode, since the semi-solid material tends to thicken and harden during the dwell time.
Another problem associated with electroheating of a biological fluid, is that the fluid contacts the electrode. The electrodes are usually made of graphite, which is preferable to metal because metal ions can dissolve in the contacting fluid, whereas graphite does not. Nevertheless, even with graphite electrodes, there is an electrolytic reaction with the fluid, and the fluid becomes reduced. Although in some cases this can be beneficial, such as in recovery of oxidized vitamin C in electroheated orange juice, nevertheless in some cases this may not be desirable.
Applicant/assignee's U.S. Pat. No. 6,088,509, the disclosure of which is incorporated herein by reference, describes an electrolytic bridge that solves the abovementioned problems. The electrode of the electrolytic bridge does not come into direct contact with the flowable material which is to be heated. The electrode is generally conical in shape and defines a chamber which is also conical. The chamber is filled with an electrolytic solution which wets a porous, electrically non-conductive conduit through which the flowable material is passed. Electrical current passes from the electrode through the electrolytic solution to the conduit and into the flowable material, thereby electroheating the material.
Due to the conical shape of the electrode and chamber, the current is not concentrated at the upstream base of the chamber, but rather is distributed along the length of the conduit and the electrode, thereby ensuring a relatively low current density. Most preferably, the electrolytic solution is chosen to have an electrical conductivity such that, taking into consideration the electrical conductivity of the flowable material, there is generally an equal distribution of current through the flowable material along the entire length of the conduit. There is a short dwell time because the flowable material flows through a cylindrical conduit rather than through a cone.
However, even with the electrolytic bridge of U.S. Pat. No. 6,088,509, certain coagulation problems can still occur. Coagulation of proteins occurs when a particle is caught and is delayed in the electroheater. The high electrical current heats the snagged particle up to ignition temperatures. Once the particle burns, it forms an obstacle to the flow and more proteinaceous matter coagulates and burns. This can also lead to arcing, since the carbon is more conductive than the flowable material, meaning that the current prefers to flow through the carbon. When using plastic tubes, the hot carbon burns the plastic.
In order to prevent the risk of arcing, the material should flow very fast without any obstacles. Even a change in diameter of the tube through which the material flows can lead to deposition of conductive proteins that eventually tend to burn. Although the porous tube and electrolytic bridge of U.S. Pat. No. 6,088,509 enable a straight line flow with no appreciable change in diameter, nevertheless the porous tube must still be connected to a non-conductive tube, and some flowable material can get caught at the connection between the two tubes. After processing a large amount of material, there can be an accumulation of caught material which then burns.